Re: [PATCH] arm64: remove page granularity limitation from KFENCE

[Liu Shixin <liushixin2@huawei.com>]

On 2021/9/18 19:50, Alexander Potapenko wrote:
> On Sat, Sep 18, 2021 at 10:10 AM Liu Shixin <liushixin2@huawei.com> wrote:
>> Currently if KFENCE is enabled in arm64, the entire linear map will be
>> mapped at page granularity which seems overkilled. Actually only the
>> kfence pool requires to be mapped at page granularity. We can remove the
>> restriction from KFENCE and force the linear mapping of the kfence pool
>> at page granularity later in arch_kfence_init_pool().
> There was a previous patch by Jisheng Zhang intended to remove this
> requirement: https://lore.kernel.org/linux-arm-kernel/20210524180656.395e45f6@xhacker.debian/
> Which of the two is more preferable?
The previous patch by Jisheng Zhang guarantees kfence pool to be mapped at
page granularity by allocating kfence pool before paging_init(), and then map it
at page granularity during map_mem().

The previous patch has a problem: Even if kfence is disabled in cmdline, kfence pool
is still allocated, which is a waste of memory.

I'm sorry for sending it repeatly, and I have no idea how to limit the email format
to TEXT/PLAIN.

thanks.

>> Signed-off-by: Liu Shixin <liushixin2@huawei.com>
>> ---
>>  arch/arm64/include/asm/kfence.h | 69 ++++++++++++++++++++++++++++++++-
>>  arch/arm64/mm/mmu.c             |  4 +-
>>  2 files changed, 70 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/kfence.h b/arch/arm64/include/asm/kfence.h
>> index aa855c6a0ae6..bee101eced0b 100644
>> --- a/arch/arm64/include/asm/kfence.h
>> +++ b/arch/arm64/include/asm/kfence.h
>> @@ -8,9 +8,76 @@
>>  #ifndef __ASM_KFENCE_H
>>  #define __ASM_KFENCE_H
>>
>> +#include <linux/kfence.h>
>>  #include <asm/set_memory.h>
>> +#include <asm/pgalloc.h>
>>
>> -static inline bool arch_kfence_init_pool(void) { return true; }
>> +static inline int split_pud_page(pud_t *pud, unsigned long addr)
>> +{
>> +       int i;
>> +       pmd_t *pmd = pmd_alloc_one(&init_mm, addr);
>> +       unsigned long pfn = PFN_DOWN(__pa(addr));
>> +
>> +       if (!pmd)
>> +               return -ENOMEM;
>> +
>> +       for (i = 0; i < PTRS_PER_PMD; i++)
>> +               set_pmd(pmd + i, pmd_mkhuge(pfn_pmd(pfn + i * PTRS_PER_PTE, PAGE_KERNEL)));
>> +
>> +       smp_wmb(); /* See comment in __pte_alloc */
>> +       pud_populate(&init_mm, pud, pmd);
>> +       flush_tlb_kernel_range(addr, addr + PUD_SIZE);
>> +       return 0;
>> +}
>> +
>> +static inline int split_pmd_page(pmd_t *pmd, unsigned long addr)
>> +{
>> +       int i;
>> +       pte_t *pte = pte_alloc_one_kernel(&init_mm);
>> +       unsigned long pfn = PFN_DOWN(__pa(addr));
>> +
>> +       if (!pte)
>> +               return -ENOMEM;
>> +
>> +       for (i = 0; i < PTRS_PER_PTE; i++)
>> +               set_pte(pte + i, pfn_pte(pfn + i, PAGE_KERNEL));
>> +
>> +       smp_wmb(); /* See comment in __pte_alloc */
>> +       pmd_populate_kernel(&init_mm, pmd, pte);
>> +
>> +       flush_tlb_kernel_range(addr, addr + PMD_SIZE);
>> +       return 0;
>> +}
>> +
>> +static inline bool arch_kfence_init_pool(void)
>> +{
>> +       unsigned long addr;
>> +       pgd_t *pgd;
>> +       p4d_t *p4d;
>> +       pud_t *pud;
>> +       pmd_t *pmd;
>> +
>> +       for (addr = (unsigned long)__kfence_pool; is_kfence_address((void *)addr);
>> +            addr += PAGE_SIZE) {
>> +               pgd = pgd_offset(&init_mm, addr);
>> +               if (pgd_leaf(*pgd))
>> +                       return false;
>> +               p4d = p4d_offset(pgd, addr);
>> +               if (p4d_leaf(*p4d))
>> +                       return false;
>> +               pud = pud_offset(p4d, addr);
>> +               if (pud_leaf(*pud)) {
>> +                       if (split_pud_page(pud, addr & PUD_MASK))
>> +                               return false;
>> +               }
>> +               pmd = pmd_offset(pud, addr);
>> +               if (pmd_leaf(*pmd)) {
>> +                       if (split_pmd_page(pmd, addr & PMD_MASK))
>> +                               return false;
>> +               }
>> +       }
>> +       return true;
>> +}
>>
>>  static inline bool kfence_protect_page(unsigned long addr, bool protect)
>>  {
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index cfd9deb347c3..b2c79ccfb1c5 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -516,7 +516,7 @@ static void __init map_mem(pgd_t *pgdp)
>>          */
>>         BUILD_BUG_ON(pgd_index(direct_map_end - 1) == pgd_index(direct_map_end));
>>
>> -       if (can_set_direct_map() || crash_mem_map || IS_ENABLED(CONFIG_KFENCE))
>> +       if (can_set_direct_map() || crash_mem_map)
>>                 flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
>>
>>         /*
>> @@ -1485,7 +1485,7 @@ int arch_add_memory(int nid, u64 start, u64 size,
>>          * KFENCE requires linear map to be mapped at page granularity, so that
>>          * it is possible to protect/unprotect single pages in the KFENCE pool.
>>          */
>> -       if (can_set_direct_map() || IS_ENABLED(CONFIG_KFENCE))
>> +       if (can_set_direct_map())
>>                 flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
>>
>>         __create_pgd_mapping(swapper_pg_dir, start, __phys_to_virt(start),
>> --
>> 2.18.0.huawei.25
>>
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>
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